1. Field of the Invention
The present invention relates to honeycomb structure bodies and methods of designing a honeycomb structure body having an improved structure capable of allowing exhaust gas emitted from an internal combustion engine, etc. to flow at a uniform flow speed in overall cells, and to purify exhaust gas with high efficiency.
2. Description of the Related Art
There has been known a catalyst converter equipped with a honeycomb structure body capable of purifying exhaust gas emitted from an internal combustion engine mounted on vehicles, etc. Such a catalyst converter is arranged inside of an exhaust gas pipe connected to an internal combustion engine of a vehicle, for example. In general, the honeycomb structure body has a plurality of cells formed along an axial direction of the honeycomb structure body. Each of the cells is surrounded by cell walls. Each of the cells has a straight tubular shape, and a cross section thereof is a rectangle shape, a hexagonal shape, etc. When exhaust gas having a high temperature emitted from an internal combustion engine passes through the cells of the honeycomb structure body, heat energy of the exhaust gas of a high temperature activates the catalyst supported by the cells, and the activated catalyst purifies the exhaust gas. Purified exhaust gas is discharged outside of the vehicle through the exhaust gas pipe. In general, exhaust gas passes more through a central portion than an outer portion of the honeycomb structure body, viewed in a cross section which is perpendicular to the axial direction of the honeycomb structure body.
For example, a patent document 1, Japanese patent laid open publication No. 2008-18370, discloses such a honeycomb structure body for use in a catalyst converter. The honeycomb structure body disclosed in the patent document 1 has cells and cell walls. Each of the cells is surrounded by the cell walls to make a straight tubular shape. The cells extend along an axial direction of the honeycomb structure body. In particular, the cell walls are formed in a hyperbola arrangement in which the cell walls are arranged along a plurality of hyperbolas as viewed in a cross section perpendicular to the axial direction of the honeycomb structure body. A cell density of the cells per unit area in a cross section, which is perpendicular to the axial direction of the honeycomb structure body, is reduced from a central point to the outer periphery of the honeycomb structure body. This structure increases an exhaust gas flowing area of the cells formed in the outer periphery through which exhaust gas passes, and allows the overall cells to have a uniform flow amount of exhaust gas.
However, the honeycomb structure body disclosed in the patent document 1 causes some problems. For example, because the cell walls are arranged along a plurality of hyperbolas in the honeycomb structure body, a curvature of the hyperbola becomes large at the outer periphery in a radial direction of the honeycomb structure body. This causes deformation of the cells formed in the outer periphery. The more a cell density of the cells formed in the outer periphery becomes small, the more the distortion in shape of the cells formed in the outer periphery increases. The generation of distortion in shape of the cells reduces the strength of the honeycomb structure body, and causes the exhaust gas to have non-uniform flow speed in the overall cells in the honeycomb structure body.